Golf balls have been designed to provide particular playing characteristics. These characteristics generally include initial ball velocity, coefficient of restitution (COR), compression, weight distribution and spin of the golf ball, which can be optimized for various types of players.
Generally, the hardness of a golf ball or a golf ball core is one of many factors used in designing golf balls. Typically, when a golf ball is hard, e.g., possessing a high compression value and low deformation, it typically has high COR and high initial velocity after impact with a golf club. However, a hard golf ball has a firm feel and can be difficult to control around the greens. A softer golf ball, e.g., having a lower compression value and high deformation, feels better and is easier to control with short iron clubs for greenside play.
Recent advancements in golf ball design can produce golf balls with the combination of low compression for soft “feel” and high COR for long flight distance. The COR for low compression golf balls, however, decreases at higher impact speed with golf clubs.
Solid golf balls are typically made with a solid core encased by a cover, both of which can have multiple layers, such as a dual core having a solid center and an outer core layer, or a multi-layer cover having an inner. Generally, golf ball cores and/or centers are constructed with a thermoset rubber, typically a polybutadiene-based composition. The cores are usually heated and crosslinked to create certain characteristics, such as higher or lower compression, which can impact the spin rate of the ball and/or provide better “feel.” These and other characteristics can be tailored to the needs of golfers of different abilities. From the perspective of a golf ball manufacturer, it is desirable to have cores exhibiting a wide range of properties, such as resilience, durability, spin, and “feel,” because this enables the manufacturer to make and sell many different types of golf balls suited to differing levels of ability.
Heretofore, most single core golf ball cores have had a conventional hard-to-soft hardness gradient from the surface of the core to the center of the core, otherwise known as a “positive hardness gradient.” The patent literature contains a number of references that discuss a hard-surface-to-soft-center hardness gradient across a golf ball core.
U.S. Pat. No. 4,650,193 to Molitor et al. generally discloses a hardness gradient in the surface layers of a core by surface treating a slug of curable elastomer with a cure-altering agent and subsequently molding the slug into a core. This treatment allegedly creates a core with two zones of different compositions, the first part being the hard, resilient, central portion of the core, which was left untreated, and the second being the soft, deformable, outer layer of the core, which was treated by the cure-altering agent. The two “layers” or regions of the core are integral with one another and, as a result, achieve the effect of a gradient of soft surface to hard center.
U.S. Pat. No. 3,784,209 to Berman, et al. generally discloses a soft-to-hard hardness gradient. The '209 patent discloses a non-homogenous, molded golf ball with a core of “mixed” elastomers. A center sphere of uncured elastomeric material is surrounded by a compatible but different uncured elastomer. When both layers of elastomer are concurrently exposed to a curing agent, they become integral with one another, thereby forming a mixed core. The center of this core, having a higher concentration of the first elastomeric material, is harder than the outer layer. One drawback to this method of manufacture is the time-consuming process of creating first elastomer and then a second elastomer and then molding the two together.
Other patents discuss cores that receive a surface treatment to provide a soft ‘skin’. However, since the interior portions of these cores are untreated, they have the similar hard surface to soft center gradient as conventional cores. For example, U.S. Pat. No. 6,113,831 to Nesbitt et al. generally discloses a conventional core and a separate soft skin wrapped around the core. This soft skin is created by exposing the preform slug to steam during the molding process so that a maximum mold temperature exceeds a steam set point, and by controlling exothermic molding temperatures during molding. The skin comprises the radially-outermost 1/32 inch to ¼ inch of the spherical core. U.S. Pat. Nos. 5,976,443 and 5,733,206, both to Nesbitt et al., disclose the addition of water mist to the outside surface of the slug before molding in order to create a soft skin. The water allegedly softens the compression of the core by retarding crosslinking on the core surface, thereby creating an even softer soft skin around the hard central portion.
Additionally, a number of patents disclose multilayer golf ball cores, where each core layer has a different hardness thereby creating a hardness gradient from core layer to core layer.
There remains a need, however, to achieve a single layer core that has a soft-to-hard gradient (a “negative” hardness gradient), from the surface towards the center, and to achieve a method of producing such a core that is inexpensive and efficient. A core exhibiting such characteristics would allow the golf ball designer to create products with unique combinations of compression, “feel,” and spin.